1. Field of the Invention
Exemplary embodiments of the present invention generally relate to a developing unit containing a two-component developer including magnetic carrier particles and toner particles, a process cartridge including the developing unit, and an image forming apparatus, such as a copier, printer, facsimile machine, and the like, incorporating the developing unit.
2. Discussion of the Related Art
Developing units that develop toner images for electrophotographic printing generally employ either a one-component developer or a two-component developer. While the one-component developer includes toner particles only, the two-component developer includes toner particles and magnetic carrier particles.
Such developing units include a developer bearing member for bearing the developer to convey it to a development region where the developer bearing member faces an image bearing member. The developer bearing member may include a cylindrical development sleeve, for example, constituted as a hollow cylinder the interior of which contains a magnetic field generator capable of generating a magnetic field sufficient to hold the magnetic carrier particles of the developer on the exterior perimeter surface of the development sleeve. Toner particles are then electrostatically attracted to the magnetic carrier particles. As the development sleeve rotates, the toner particles attached to the magnetic carrier particles that are held on the exterior perimeter surface of the development sleeve are conveyed to the development region and then supplied to a latent image formed on a surface of the image bearing member at the development region.
The magnetic field generator has multiple magnetic poles along a direction of rotation of the development sleeve. Examples of such magnetic field generator are a roller-shaped member having magnetic pole-forming parts magnetized by external magnetic fields, a member in which multiple magnets are held by a common holding member so that each of the magnets faces a given direction, and the like.
Developer carried on the exterior perimeter surface of the development sleeve by the magnetic force generated by the magnetic field generator is conveyed in a direction of movement of the surface of the development sleeve as the development sleeve rotates.
FIG. 1 illustrates a schematic configuration of an example of a generally known developing unit 1214, and more specifically an end-on or lateral cross-sectional view thereof. Broken lines in FIG. 1 shows distribution of magnetic flux density (absolute value) in a direction normal to a surface of a developer bearing member. This conventional developing unit 1214 is hereinafter referred to as a first conventional developing unit 1214.
The first conventional developing unit 1214 includes a developer roller 1240 that serves as a developer bearing member and includes an outer development sleeve 1241 serving as a nonmagnetic hollow body and an inner magnetic roller 1247 serving as a magnetic field generator. That is, the developer roller 1240 is formed by the hollow cylindrical development sleeve 1241 made of some non-magnetic material surrounding the magnetic roller 1247, so as to hold developer on an exterior perimeter surface of the development sleeve 1241 by a magnetic force generated by the magnetic roller 1247.
The developing unit 1214 further includes a developer container 1249 for containing developer, screw-shaped agitation/conveyance members 1242 and 1243 for agitating and conveying the developer axially along a direction of a rotary shaft of the development sleeve 1241, and a developer regulating member 1246 for regulating the thickness of a layer of developer carried on the development sleeve 1241.
The developer container 1249 is separated in a first container (i.e., a developer storing chamber) 1249A and a second container (i.e., a developer agitating chamber) 1249B. The first container 1249A is positioned lower than the development sleeve 1241 and extends in an axial direction of the development sleeve 1241. The second container 1249B is disposed adjacent the first container 1249A and also extends in the axial direction of the development sleeve 1241. The first container 1249A includes the agitation/conveyance member 1242 and the second container 249B includes the agitation/conveyance member 1243 that rotates in a direction indicated by arrow “R1” in FIG. 1. The agitation/conveyance member 1243 conveys the developer to a downstream end of the first container 1249A, which corresponds to a far or distal side in FIG. 1. The developer is then conveyed to the second container 1249B through a space or opening where the first container 1249A and the second container 1249B meet and are communicably coupled together. In the second container 1249B, the agitation/conveyance member 1242 conveys the developer to a downstream end of the second container 1249B, which corresponds to a near or proximal side in FIG. 1. Thus, the developer is circulated or recirculated within the developer container 1249.
Toner is generally supplied from a toner bottle, not shown, to the second container 1249B for replenishment, that is, replacing an amount of toner consumed for development. During conveyance of the developer, the magnetic force generated by the magnetic roller 1247 scoops up, or attracts, the developer contained in the first container 1249A, which is then supplied to the development sleeve 1241. Then, the thickness of the layer of thus-supplied developer on the development sleeve 1241 is regulated by the developer regulating member 1246, and the developer passes the development region facing an image bearing member 1012, and returns to the developer container 1249.
The magnetic roller 1247 includes five magnetic poles, which are a magnetic pole S1 for development, a magnetic pole N1 for conveyance, a magnetic pole S2 for developer release at an upstream portion, a magnetic pole S3 for developer release and attraction, and a magnetic pole N2 for regulation. Where the magnetic poles S1, S2, and S3 are implemented as south poles, for example, the magnetic poles N1 and N2 are implemented as north poles, for example.
As the development sleeve 1241 rotates in a direction indicated by arrow “R2” in FIG. 1, the developer held on the development sleeve 1241 is conveyed and then passes by positions facing the magnetic pole S3, the magnetic pole N2, the magnetic pole S1, the magnetic pole N1, and the magnetic pole S2, in this order. After passing the development region, most of the toner particles of the developer are consumed for developing toner images. Therefore, the developer is released or removed from the development sleeve 1241 to return to the developer container 1249 so that new developer can be constantly attracted to the development sleeve 1241. This action is important to provide stable development ability. That is, this action is important to prevent developer carryover or residual retention, in which developer with fewer toner particles remains on the development sleeve 1241 even post-development to be conveyed continuously to the development region again.
When the magnetic pole S2 and the magnetic pole S3 having an identical polarity are disposed adjacent to each other, a developer-releasing region P is formed between the magnetic poles S2 and S3 in the developing unit 1214 shown in FIG. 1 that exerts a release force to cause the developer carried by the development sleeve 1241 to move away from the development sleeve 1241 and toward the first container 1249A of the developer container 1249. That is, the magnetic force generated by the magnetic poles S2 and S3 releases the developer from the development sleeve 1241 in the developer-releasing region P, so that the developer is removed from the development sleeve 1241 and mixed with the developer in the first container 1249A of the developer container 1249.
The first conventional developing unit 1214 shown in FIG. 1 has a polarity inversion point Q on the development sleeve 1241, located within a region extending from the developer-releasing region P to a developer-regulating region where the developer regulating member 1246 regulates the developer scooped up to the development sleeve 1241 by the magnetic force generated by the magnetic pole S3. Developer density is high around the polarity inversion point Q because the magnetic force exerted on the developer is relatively strong and a magnetic flux density in a direction normal to the development sleeve 1241 is too small to form a magnetic brush. Accordingly, even if some developer remains on the development sleeve 1241 without being removed therefrom in the developer-releasing region P, such residual developer can be released or scraped off by the high-density developer held in the vicinity of the polarity inversion point Q. For this reason, this conventional developing unit 1214 can effectively prevent developer carryover.
However, such a continuous high-density state of developer in the vicinity of the polarity inversion point Q imposes a constant mechanical stress on the developer particles, causing them to deteriorate. Therefore, an amount of torque to drive the agitation/conveyance member 1243 of the first container 1249A has to be increased and the agitation/conveyance member 1243 has to be more rigid in strength and larger in size, which can lead to an increase both in cost and in size of the first conventional developing unit 1214.
Further, since the developer is subject to a great amount of stress, a speed of progression of implantation of external additives from the toner into the surface of each carrier particle and abrasion of a surface layer film of each carrier particle, both of which are undesirable, may be accelerated. These actions easily can degrade toner chargeability and powder flowability of developer, which in turn can make it difficult to maintain good image quality over an extended period of time. Since the powder properties of developer can degrade easily, an amount of developer conveyed to the development region may decrease especially when the ability of the development sleeve 1241 to convey developer has deteriorated, and good image quality cannot be maintained for an extended period of time.
FIG. 2 illustrates a schematic configuration of another example of a generally known developing unit 1314. This known developing unit 1314 is referred to as a second conventional developing unit 1314. The second conventional developing unit 1314 reduces an amount of stress on the developer. The second conventional developing unit 1314 shown in FIG. 2 is similar to the first conventional developing unit 1214 shown in FIG. 1, except that a single magnetic pole capable of performing removal, attraction, and regulation of developer simultaneously is provided in the vicinity of a developer regulating member 1346, instead of the known magnetic poles S3 and N2 shown in FIG. 1.
Similar to the first conventional developing unit 1214, the second conventional developing unit 1314 includes a developer roller 1340 that serves as a developer bearing member and is disposed facing the image bearing member 1012, and includes an outer development sleeve 1341 serving as a nonmagnetic hollow body and an inner magnetic roller 1347 serving as a magnetic field generator. The development unit 1314 further includes a developer container 1349 for containing developer, screw-shaped agitation/conveyance members 1342 and 1343, and the developer regulating member 1346 for regulating the thickness of layer of developer carried on the development sleeve 1341 that rotates in a direction indicated by arrow “R2” in FIG. 2. The developer container 1349 is separated into a first container (i.e., a developer storing chamber) 1349A and a second container (i.e., a developer agitating chamber) 1349B.
According to the second conventional developing unit 1314 shown in FIG. 2, the developer that cannot be scooped up by the magnetic force of the magnetic pole N3 may fall to the agitation/conveyance screw 1343 (which rotates in a direction indicated by arrow “R1” in FIG. 2) in a region upstream from the developer-regulating region where the developer regulating member 1346 regulates the thickness of layer of developer in a direction of conveyance of developer by the development sleeve 1341 of the developing roller 1340. (Hereinafter, “upstream” and “downstream” indicate an upstream side and downstream side from a given specific position in a direction of conveyance of developer by the development sleeve 1341, respectively.) Such an arrangement prevents a large body of developer from accumulating in the region, thereby reducing the stress on the developer.
Although not disclosed in the first conventional developing unit 1214 and the second conventional developing unit 1314, the developer tends to accumulate in an area from at least a downstream part of the developer-releasing region P that is located upstream from the developer-regulating region to the developer-regulating region in the developing units 1214 and 1314. Whit this arrangement, in the second conventional developing unit 1314, while the developer released from the development sleeve 1341 in an upstream part of the developer-releasing region P may fall onto the agitation/conveyance screw 1343, the developer remaining on the development sleeve 1341 after passing the downstream part of the developer-releasing region P may be taken in developer accumulated in the area to be released or removed therefrom. That is, similar to the first conventional developing unit 1214 shown in FIG. 1, the developer remaining on the development sleeve 1341 can be removed or scraped off by the developer in the developer container 1349. For this reason, the second conventional developing unit 1314 can effectively prevent the carryover of developer.
However, the above-described configuration, in which the developer in the developer container 1349 is used for removing the developer remaining on the development sleeve 1341, may impose a certain amount of mechanical stress on the developer when the developer on the development sleeve 1341 is scraped therefrom. In light of market demands to reduce stress on the developer as much as possible, it is also desired to reduce the above-described stress on the developer when scraping the developer off the development sleeve 1341.
Consequently, the present inventors have conducted extensive research designed to eliminate stress on the developer when scraping it off a development sleeve, and as a result have developed a developing unit 1414 as shown in FIG. 3.
As illustrated in FIG. 3, the developing unit 1414 includes a developing roller 1440, a developer container 1449, and a developer regulating member 1446. The developing roller 1440 serves as a developer bearing member, includes an outer development sleeve 1441 serving as a nonmagnetic hollow body and an inner magnetic roller 1447 serving as a magnetic field generator, and is disposed facing the image bearing member 1012. The developer container 1449 includes two container sections, a first container 1449A and a second container 1449B. The first container 1449A includes a screw-shaped agitation/conveyance member 1443, which rotates in a direction indicated by arrow “R1” in FIG. 3, and the second container 1449B also includes screw-shaped agitation/conveyance member 1442, which rotates in a direction indicated by arrow “R2” in FIG. 3.
In the developing unit 1414, the developing roller 1440 is shifted upward in relation to the developer container 1449 as shown in FIG. 3 so that the developer-releasing region P on the development sleeve 1441 does not contact the top surface of developer in the developer storing chamber 1449A when the development sleeve 1441 rotates in a direction indicated by arrow “R3” in FIG. 3. With this configuration of the developing unit 1414, even though some developer might remain in the developer-releasing region P on the development sleeve 1441, that developer is not scraped off the development sleeve 1441 by the developer stored in the developer storing chamber 1449A, and therefore is not stressed due to scraping.
However, the inventors have found that the developing unit 1414 can cause the following problems.
As described above, the configuration of the developing unit 1414 prevents the developer in the developer-releasing region P on the development sleeve 1441 from contacting the developer stored in the developer storing chamber 1449A. With this configuration, the developer released from the development sleeve 1441 in the developer-releasing region P is subject to the action of a rotative force or torque of the development sleeve 1441 and a release force of the magnetic force generated by the magnetic poles, and consequently flies into the developer container 1449 to be taken and mixed into the developer therein.
Adjacent to and on the downstream side of the developer-releasing region P, a developer-attracting region R is provided to attract the developer with the magnetic force generated by the magnetic pole N3 for developer release, attraction, and regulation. The developer released in the developer-releasing region P, especially an end part thereof, is also subjected to the torque of the development sleeve 1441 and consequently moves toward a downstream side of the developer-releasing region P, which can cause the developer to fly toward the developer-attracting region R while moving away from the development sleeve 1441. Therefore, the developer flying toward the developer-attracting region R after being released from the development sleeve 1441 is affected by the magnetic force generated by the magnetic pole N3 and reattaches to the development region by being taken in with other developer attracted to the developer-attracting region R or by directly adhering or attaching to the developer-attracting region R. As with the above-described developer carryover, such developer reattachment hinders, stable image development, but with this difference:
Whereas the developer carryover causes unevenness in image density with streaks that may extend in a circumferential direction of the development sleeve 1441, the developer reattachment causes unevenness in image density with spots. However, both the developer carryover and the developer reattachment can degrade image quality.
Further, the developer in the downstream part of the developer-releasing region P contacts the surface of the developer stored in the developer storing chamber 1349A and can act as a wall to protect the developer released from the upstream part of the developer-releasing region P, so that the developer may not be taken by the developer in the developer attracting region R and/or may directly adhere to the developer attracting region R, for example.
Further, in the developing unit shown in FIG. 3, the developer remaining on the development sleeve 1441 cannot be scraped off by the developer stored in the developer storing chamber 1449A before the developer is conveyed to the developer attracting region R. Consequently, developer carryover can occur easily.